1. Field of the Invention
This invention generally relates to a linear motion guide unit, and, in particular, to a double linear motion guide unit assembly suitable for use in semiconductor manufacturing apparatuses, electronic business machines and various other devices, such as reading devices.
2. Description of the Prior Art
FIG. 4 illustrates in perspective view a typical prior art linear motion guide unit, which generally includes a rail A which is formed with a pair of inner guide grooves on its opposite side surfaces, a slider B mounted on the rail A in a straddling manner and having a pair of outer guide grooves E each located opposite to a corresponding one of the inner guide grooves C, and a plurality of rolling members F, or balls in the illustrated example, interposed between the rail A and the slider B as partly received in the paired inner and outer guide grooves C and E. In the structure shown in FIG. 4, the slider B is provided with a pair of endless circulating paths, each including a load path section, a return path section and a pair of curved connecting path sections connecting the corresponding ends of the load and return path sections, and the load path section is defined by the corresponding outer guide groove E.
With the above-described structure, since the balls E may roll along the associated endless circulating path indefinitely, the slider B may move along the rail A as long as the rail A extends. Those balls E which are located in the load path section are partly received in both of the inner and outer guide grooves C and E so that a rolling contact is provided between the rail A and the slider B. In order to provide a high precision, the linear motion guide unit shown in FIG. 4 is typically so structured that there is no room for the slider B to move in a transverse direction indicated by the double arrow M normal to the longitudinal axis of the rail A.
Using the linear motion guide unit shown in FIG. 4, a double linear motion guide unit assembly is constructed as illustrated in FIG. 5. The illustrated double linear motion guide unit assembly includes a pair of rails G and H which are fixedly mounted on a base I as spaced apart from each other and extending in parallel. Each of the rails G and H corresponds to the rail A in the structure shown in FIG. 4. The present assembly also includes a first pair of front and rear sliders J and K which are slidably mounted on the rail G and each of these sliders J and K corresponds to the slider B in FIG. 4. In addition, the present assembly also includes another pair of front and rear sliders L which are slidably mounted on the rail H and each of them also corresponds to the slider B in FIG. 4. The present assembly also includes a sliding table which is commonly mounted on these four sliders J, K and L as fixedly attached thereto. The sliding table is thus defines as a moving table for supporting thereon any desired portions of movable parts in semiconductor manufacturing devices and electronic business machines in which the present assembly is incorporated.
As described before, since there is no room for relative motion between the rail A and the slider B in the transverse direction as indicated by the double arrow M in FIG. 4, there is no room for relative motion between the sliding table and the rails G and H in the transverse direction as indicated by the double arrow M in FIG. 5. Although such a tight lateral constraint is not a problem when the linear motion guide unit shown in FIG. 4 is used alone, a significant problem could arise when two or more of the linear motion guide unit of FIG. 4 were used side-by-side as in the double linear motion guide unit assembly shown in FIG. 5. That is, since no tolerance is allowed in the transverse direction, the two rails G and H must be laid down in parallel at an extremely high accuracy, typically on the order of several tens of microns at maximum. As a result, difficulty will be encountered in building the assembly shown in FIG. 5 and the cost tends to be pushed up. In addition, the error in the parallel arrangement between these two rails also could increase due to an increase in temperature, and, thus, its scope of applications is rather limited, or special means must be provided for compensating for the temperature effects.